A variety of techniques are currently used to transfer lithographically defined patterns to underlying semiconductors for electronic and optoelectronic applications. Such techniques include solvent etching, plasma etching and ion beam assisted etching. Plasma etching processes are the most extensively used of these techniques.
The most commonly used form of plasma etching is reactive ion etching (RIE). However, there is an urgent need for a substantial reduction in energetic ion bombardment damage and charge-induced damage occurring during RIE. While bombardment damage can be reduced by decoupling the plasma generation means from the processing electrode (typically done with new high-density plasma tools), it cannot be completely avoided as this would require a decrease in the translational energy of the ions to below 20 eV, where space-charge repulsion adversely affects directionality. Furthermore, mitigation of charge-induced damage is inherently more difficult because the very nature of plasmas is based on charged particles.
Winters and Coburn have suggested that neutral fluxes of reactive species with excess translational or internal energy, created using supersonic sources, may provide "new and useful etching methods . . . " Winters, et al. Surface Science Reports 14 161 (1992). NASA Tech Briefs, February 1993, include a description of the use of atomic oxygen to anisotropically etch photoresists from the surface of semiconductor workpieces, see Energetic Atoms Would Etch Photoresists Anisotropically, MSC-21631, the contents of which are incorporated herein by reference.
U.S. Pat. No. 4,894,511 describes an apparatus and method for creating beams of atomic oxygen which are said to be useful for surface erosion, surface coating and the like. Hydrogen, fluorine and chlorine, among others, are mentioned as potentially useful for the same applications. Similarly, U.S. Pat. No. 4,780,608 to Cross et al. describes an apparatus in which laser radiation is used to form a plasma, which, in turn, is used to create a continuous collimated beam of oxygen atoms, in certain cases as mixtures with other gases. This beam is said to be useful for studying the effects of high kinetic energy oxygen atom bombardment on a target material. Each of these patents is also incorporated herein by reference.
Holtzclaw et al., Infrared Emission From The Reaction of High-Velocity Atomic Oxygen With Graphite and Polyethylene, J. Geophysical Research, Vol. 95, No. A4, pages 4147-4153, Apr. 1, 1990, describes an apparatus useful for generating a beam of oxygen atoms. This reference is incorporated herein by reference as background to the present invention.
Any barrier to etching could be overcome by sufficient translational energy of the etchant species in the beam. However, if energetic neutral beams are to be useful, the problem of profile control must be solved. Some progress has already been made toward the goal of profile control during etching with neutral beams. Anisotropic etching of semiconductors (Si, GaAs) with a supersonic beam of neutral Cl.sub.2 molecules has been demonstrated. In addition, work from the space environmental effects community has shown that a beam of energetic oxygen atoms can etch 0.3 .mu.m-wide features in an organic polymer with insignificant undercutting. An observation common to both types of etching is the existence of a dependence of the etch rate on initial collision energy. Typically, when energy dependent etching data are fit with an Arrhenius-like expression, activation energies in the range of 0.2-0.4 eV are derived. In the case of silicon etching with Cl.sub.2, a threshold to etching of 2.0 eV has been observed. These findings indicate that practical etching with neutral beams may only be achieved with hyperthermal translational energies greater than 2 eV, however, in order to avoid bombardment damage, energies should be kept below the lattice displacement damage threshold, which for silicon is 12.9 eV. The hyperthermal regime of 2-12 eV is not easily accessible.
Conventional supersonic and effusive beam techniques, as well as a laser blow-off technique, have been used to study the interaction of neutral species with semiconductor surfaces. Although these experiments reveal much about the interaction mechanisms under the conditions studied, the practical range of incident-kinetic energies is limited to a few electron volts or less, and the combination of energy and incident flux usually results in too low an etch rate for a study of etch profiles. Furthermore, achievable exposure areas are less than 1 cm.sup.2, making it difficult to fabricate even a prototype device for testing.
Thus, a technique is needed not only to accelerate selected neutral species to these kinetic energies but also to generate a collimated beam of these species over an area that would permit fabrication of a real chip (&gt;5 cm.sup.2).